The present invention relates to a method for hardening the surfaces of work pieces made of stainless steel, as well as to a molten salt bath for realizing this method.
As a result of its excellent corrosion resistance, stainless steel is used for chemical apparatuses, in food technology, in the petro-chemical industry, in offshore areas, for the construction of ships and airplanes, in the field of architecture, for home construction and equipment manufacturing, as well as in many other areas of industry.
Stainless steel is considered to be corrosion-resistant if at least 13 weight % of chromium is added by alloying to an iron material. In most cases the iron alloy also contains nickel, titanium and molybdenum, for example as described in Stahl Merkblatt 821 “Edelstahl Rostfrei—Eigenschaften Informationsstelle Edelstahl,” PF 102205, 40013 Düsseldorf www.edelstahl-rostfrei.de [Steel Leaflet 821, “Corrosion-Resistant Stainless Steel—Information Source for the Properties of Stainless Steel,” PF . . . ] and in P. ümpel et al. Rostfreie Stähle [Rust-Resistant Steels], Expert Publishing House, Volume 349, Renningen Malmsheim 1998.
Typical austenitic stainless steels are the steel alloys 1.4301 or 1.4571 with the following compositions:                1.4301: C 0.05; Si0.5; Mn 1.4; Cr 18.5; Ni 9.5 weight %.        1.4571: C 0.03; Si 0.5; Mn 1.7; Cr 17.0; Ni 11.2; Mo 2.2; Ti 0.1 weight %.        
If the chromium content amounts to less than 13 weight %, the steel is generally not sufficiently corrosion-resistant to be considered “stainless steel.” The metallic chromium content of the steel is thus an important criterion for the corrosion-resistance, as mentioned in particular in the aforementioned publication by P. Gümpel.
One great disadvantage of most of the commonly used stainless steel types, such as 1.4301, 1.4441, 1.4541 or 1.4575, is that these are relatively soft steels and that their surfaces can consequently be scratched by hard particles such as dust, sand and the like. Most types of stainless steel, apart from the so-called martensitic stain steels, cannot be hardened with the aid of physical processes such as annealing and chilling. The low surface hardness frequently stands in the way of a use of the stainless steel. A further disadvantage of most types of stainless steel is the strong tendency to corrosion seizing, meaning the fusing of two surfaces that slide against each other as a result of adhesion.
To counter this problem, it is known to subject work pieces made from stainless steel to a thermo-chemical treatment. During this treatment, the stainless steel surface is enriched with nitrogen through the process of nitrating or nitro-carbureting in a gas atmosphere (ammonia atmosphere), in plasma (nitrogen/argon atmosphere) or in the molten salt bath (using molten cyanates), wherein iron nitrides and chromium nitrides form. The resulting layers are formed from the material itself, meaning they are not deposited from the outside, in contrast to galvanic or physical layers, and therefore have extremely high adhesive strength. Depending on the length of treatment, hard layers form, which have a thickness ranging from 5 to 50 μm. The hardness of such nitrated or nitro-carbureted layers on stainless steel reaches values exceeding 1000 units on the Vickers Hardness Scale because of the high hardness of the resulting iron nitrides and chromium nitrides.
The problem with a practical use of such nitrated or nitro-carbureted layers on stainless steel is that these layers are hard, but also lose their corrosion resistance as a result of the relatively high treatment temperature, which is in the range of 580° C. during the nitrating or the nitro-carbureting process. At this temperature, the diffused-in elements nitrogen and carbon form stable chromium nitrides (CrN) and/or chromium carbides (Cr7C3) with the chromium in the surface region of the component. In this way, the free chromium that is absolutely necessary for the corrosion resistance is removed from the stainless steel matrix up to a depth of approximately 50 μm below the surface and is converted to chromium nitride or chromium carbide. The component surface becomes hard because of the forming of iron nitride or chromium nitride, but is also subject to corrosion. During the use of the work piece, these types of layers become quickly worn down and/or are eroded because of corrosion.
The following processes are known for avoiding this problem.
It is known that the surface hardness of stainless steel can be improved through depositing galvanic layers, e.g. through nickel-plating, or by depositing physical layers, e.g. with the aid of PVD coating (physical vapor deposition). In the process, however, a material foreign to the species is deposited on the steel surface. The surface in contact with the medium causing the wear or corrosion is no longer the steel surface itself. As a result, there are problems with the adherence and corrosion-resistance. These processes are therefore not widely used for improving the hardness and wear-resistance of stainless steel.
A hard and simultaneously corrosion-resistant layer can be generated thermo-chemically with the aid of the so-called kolsterizing on stainless steel. This process is mentioned, for example in “Kolsterisieren—korrosionsfestes Oberflächenhärten von austenitischem rostfreiem Stahl”—Informationsblatt der Bodycote Hardiff bv [“Kolsterizing—Corrosion-Resistant Surface Hardening of Austenitic Rust-Resistant Steel”—Information Leaflet by the company Bodycote Hardiff bv], Parimariboweg 45, NL-7333 Apeldoorn; at info@hardiff.de. However, the requirements for carrying out this process are not described in the patent literature or in the generally accessible scientific literature. Components treated with this process have a hard, wear-resistant layer with a thickness ranging from 10 to 35 μm while the corrosion-resistance of the basic material is retained. Kolsterized components cannot be heated above 400° C. since they otherwise lose their corrosion resistance.
With the aid of plasma nitrating, as described in H. J. Spies et al., Mat. Wiss. u. Werkstofftechnik, 30 (1999) 457-464” [H. J. Spies et al. Material Knowledge and Material Technology, 30] and in Y. Sun, T. Bell et al., “The Response of Austenitic Stainless Steels to Low Temperature Plasma Nitriding,” Heat Treatment of Metals, 1999.1, p. 9-16, or with the aid of low-pressure carburizing at low temperatures, e.g. as is described in D. Günther, F. Hoffmann, M. Jung, P. Mayr: “Oberflächenhärtung von austenitischen Stählen unter Beibehaltung der Korrosionsbeständigkeit,” Härterei-Techn. Mitt. 56 (2001) 74-83” [D. Gunther, F. Hoffmann, M. Jung, P. Mayr: “Surface Hardening of Austenitic Steels While Retaining the Corrosion Resistance,” Hardening Techn. Inform., 56 (2001) 74-83], it is possible to generate an over-saturated solution of nitrogen and/or carbon in the surfaces of stainless steel components, which has the desired properties, meaning the desired higher hardness with unchanged corrosion-resistance.
However, both processes require high apparatus expenditure and high investment and energy costs. Specially trained and in most cases even scientifically trained personnel are needed for operating the equipment.